1. Field of the Invention
This invention relates to oscillators of the type having a number of delay cells interconnected in a closed ring configuration. More particularly, this invention relates to such oscillators wherein the delay is controllable by a signal so as to vary the frequency of oscillation.
2. Description of the Prior Art
Ring oscillators have been used for many years. Modern designs have used MOS inverters as delay cells, and the delay has been controlled by setting the flow of current through the inverter. Commonly, each inverter comprises a series-connected pair of complementary MOS devices, with the current therethrough being produced by a current source controlled by an input signal.
One of the problems with such prior art ring oscillators is that the gain of the inverters varies with the level of current through the inverter as the oscillator input signal changes. This gain variability may cause the oscillator to stop if the gain drops too low, or may cause associated circuitry such as a phase lock loop to become unstable if the gain becomes too high. Another problem is that prior art oscillators of this type typically develop relatively large oscillatory voltage swings so that the oscillator frequency would be undesirably low due to the time required to charge up and discharge the parasitic capacitances associated with the MOS devices.